Generally, in large-capacity power conversion devices, converter output is high voltage or large current. Therefore, many large-capacity power conversion devices are configured with a plurality of converter cells multiplexed in series or parallel. Multiplexing of converter cells realizes not only increase in converter capacity but also reduction in harmonics contained in an output voltage waveform by combining outputs thereof. As a result, harmonic current flowing into a system can be reduced.
As one of means for multiplexing converter cells, there is a multilevel converter in which outputs of a plurality of converter cells are connected in cascade, and one of such multilevel converters is a modular multilevel converter.
Each of arms of a modular multilevel converter is composed of a plurality of converter cells connected in cascade.
A first arm and a second arm for each of phases of the conventional modular multilevel converter each have chopper cells (converter cells) and a reactor. Each chopper cell includes two power semiconductor switching elements (hereinafter referred to as switching elements) connected in series to each other, and a DC capacitor connected in parallel thereto. In each of the first arm and the second arm, substantially the same number of chopper cells are connected in cascade via their respective output ends.
Control for each phase of the conventional modular multilevel converter includes: average value control of causing the average value of voltage values of all DC capacitors to follow a DC capacitor voltage command value; individual balance control for causing the voltage value of each DC capacitor to follow the DC capacitor voltage command value; and arm balance control for causing the average value of voltage values of all the DC capacitors in the first arm and the average value of voltage values of all the DC capacitors in the second arm to coincide with each other. A circulating current which circulates in the modular multilevel converter without flowing to the outside of the modular multilevel converter is controlled, and a voltage command value is calculated so as to control to control an AC current for each phase and a DC voltage command value is calculated so as to control a DC output terminal (e.g., Patent Document 1 and Non-Patent Document 1).
Regarding a power supply required for driving the switching elements constituting each chopper cell of the conventional modular multilevel converter, a self-feeding circuit is provided which obtains power from the DC capacitor of the converter cell, thereby to omit supply of power from the outside (e.g., Patent Document 2).
Further, when overvoltage of the DC capacitor occurs in the conventional power conversion device, an electric discharge circuit in which a resistor and a switch are connected in series is connected in parallel to the DC capacitor, and a current is caused to flow through the resistor by setting the switch of the electric discharge circuit in a conductive state, whereby electric energy stored in the DC capacitor is consumed to reduce the potential of the voltage of the DC capacitor (e.g., Patent Documents 3 and 4).